In recent years, with the advance of high-speed, large-capacity optical fiber communication systems, high-speed optical modulators using optical waveguide devices, which are typified by external modulators (optical modulators based on the external modulation system), are being put to practical use and widely used in the art.
FIG. 1 is a cross-sectional view of a conventional optical modulator. The optical modulator 10 shown in FIG. 1 has coplanar waveguide (CPW) modulation electrodes for modulating light guided through an optical waveguide. Specifically, the optical modulator 10 comprises a substrate 1 in the form of an X-cut plate of lithium niobate, Mach-Zehnder optical waveguides 2 formed directly beneath a principal surface IA of the substrate 1 by titanium diffusion, a buffer layer 3 of silicon oxide formed on the principal surface IA, and a central electrode 4 and ground electrodes 5-1, 5-2 formed on the buffer layer 3.
FIG. 2 is a cross-sectional view of another conventional optical modulator. The optical modulator 20 shown in FIG. 2 has coplanar waveguide (CPW) modulation electrodes for modulating light guided through an optical waveguide. Specifically, the optical modulator 20 comprises a substrate 11 in the form of an Z-cut plate of lithium niobate, Mach-Zehnder optical waveguides 12 formed directly beneath a principal surface 11A of the substrate 11 by titanium diffusion, a buffer layer 13 of silicon oxide formed on the principal surface 11A, and a central electrode 14 and ground electrodes 15-1, 15-2 formed on the buffer layer 13.
In the optical modulators 10, 20 shown in FIGS. 1 and 2, the buffer layers 3, 13 are provided for the purpose of increasing speed matching between the light guided through the optical waveguides 2, 12 and a microwave applied to the modulating electrodes.
However, in the optical modulators 10, 20 shown in FIGS. 1 and 2, the buffer layers 3, 13 included in the substrates 1, 11 are responsible for a DC drift that is produced. Furthermore, since modulation signals are applied from the modulation electrodes through the buffer layers 3, 13 to the light guided through the optical waveguides 2, 12, a substantial modulation signal voltage applied to the light is reduced. For effectively energizing the optical modulators 10, 20, it is necessary to apply a relatively high voltage to the modulation electrodes, despite the demand for reduced drive voltages.
In the optical modulator 20 shown in FIG. 2, since the optical waveguides 12 are positioned asymmetrically with respect to the central electrode 14, the chirp increases, failing to make long-distance transmission.
The present invention provides a method of manufacturing an optical modulator having a novel arrangement which achieves speed matching without a buffer layer and which is free from the above problems.